Thousands of species of bony fishes are less than a few centimeters long as adults. Among the smallest is the endangered dwarf pygmy goby (Pandaka pygmaea). Adult males reach just 15 mm (0.6 in.), and adult females reach only about 9 mm (0.4 in.).
Some species can reach tremendous sizes - much larger than a human.
- The longest bony fish is the oarfish (Regalecus glesne), which can reach 11 m (36 ft.).
- Among the heaviest of the bony fishes is the common ocean sunfish (Mola mola), which lives throughout warm and temperate seas worldwide. A large sunfish can reach 3.3 m (10.8 ft.) and 2,300 kg (5,071 lb.).
- Many sturgeons (family Acipenseridae) grow very large. The largest is the beluga sturgeon (Huso huso), which inhabits the Caspian, Black, and Adriatic Seas and can reach 5 m (16.4 ft.) and 2,000 kg (4,409 lb.).
- Black marlin (Makaira indica) reach 4.7 m (15.4 ft.) and 750 kg (1,653 lb.).
- The European wels catfish (Silurus glanis) reaches 5 m (16.4 ft.) and about 300 kg (661 lb.).
Bony fishes show great variety in body shape, but the "typical" fish body shape is roughly cylindrical and tapering at both ends. This characteristic fusiform shape is quite energy efficient for swimming. Compared to other body shapes, this body shape creates less drag (the opposing force an object generates as it travels through water).
Various species of fishes deviate from the fusiform body shape in three ways: compression, depression, and elongation.
- A laterally compressed (flattened, side-to-side) body shape is common in bony fishes that live in dense cover or within coral reefs. Butterflyfishes (family Chaetodontidae) are an example of bony fishes with a laterally compressed body shape.
- A depressed (flattened, top-to-bottom) body shape is common in bottom-dwelling fishes. Goosefishes (family Lophidae) and batfishes (family Ogcocephalidae) are examples of bony fishes with a depressed body shape.
- The body shape of an eel (for example, the morays, family Muraenidae) is an extreme example of an elongated shape.
The body shape of some species differs from or combines features of these typical fish body forms. Examples include boxfishes (family Ostraciidae ), ocean sunfishes (family Molidae), seahorses (Hippocampus spp.), the weedy seadragon (Phyllopteryx taeniolatus), and the leafy seadragon (Phycodurus eques).
Most fish species have pigmentation.
- Pigment is mostly contained in cells called chromatophores. Most fishes can contract and expand their chromatophores to change colors.
- Reflective cells called iridocytes can change color rapidly.
- Because the different wavelengths of light are absorbed at various depths, fishes may appear a different color underwater than at the surface.
- Some fish, such as the ghost glass catfish (Kryptopterus bicirrhis), lack pigmentation.
Coloration may camouflage a fish.
- Most species of fishes are countershaded: the dorsal (top) surface is darker than the ventral (underneath) surface. When light comes from above, the animal appears inconspicuous. The dorsal side of a countershaded fish blends in with the dark ocean depths or ocean bottom when viewed from above. The ventral side blends in with the lighter surface of the sea when viewed from below. A countershaded fish is harder for predators and prey to spot.
- Some fish are colored so that they blend in with their environment. Many bottom dwelling fishes match the substrate and even change color when they move to a new location. The northern pike's (Esox lucius) colors blend in with weedy areas where it lurks in wait for prey.
Some fishes show disruptive coloration. Their colors and pattern obscure the outline of the fish by contradicting the animal's body shape.
Highly distinctive elements may confuse predators. For example, some fish have a false eyespot that can fool a predator into striking in the wrong direction, allowing the fish to escape.
In some species, coloration serves as advertisement to other animals.
- Some fishes rely on coloration for species recognition and sexual distinction. The stoplight parrotfish (Sparisoma viride) female and male are completely different colors, although they are similar in shape and size. Some species of fishes become brighter in color during breeding season to attract potential mates.
- In some species, coloration may trigger behavior. After establishing a territory, the male stickleback's (family Gasterosteidae) belly turns red. He then actively defends his territory only from other fish with red bellies, notably other male sticklebacks.
- A garibaldi's (Hypsypops rubicundus) bright orange color warns other fishes that the garibaldi will defend its territory.
Some fish change color.
- Some species change color and markings as they grow from juveniles to adults. Juvenile garibaldi (Hypsypops rubicundus), for example, are dark orange with bright blue spots; adults are bright orange.
- Fish of some species can change sex, which is accompanied by color change. Examples include the angelfishes (Family Pomacanthidae) and most species of wrasses (Family Labridae).
- Some color change may be rapid and temporary. Alarmed fish, for instance, often change color. Some bottom-dwelling fishes change color almost instantly to match the substrate.
Some fish bioluminate (emit light).
- Certain pigments (called luciferins) emit light when oxidized.
- Some fish produce light in luminescent organs or in cells called photophores. In some fish, it is light-producing bacteria that live in or on the fish that actually produce the light.
- Depending on species, bioluminescence may attract mates, deter or confuse predators, attract prey, or act as "headlights" to help a fish see in the dark.
All fishes have fins. Bony fish families show various degrees of fin fusion and reduction.
Fins help stabilize or propel a fish in the water.
Except in the lungfishes and the coelacanth, fins lack bones. In Actinopterygians, fins are supported by structures called rays.
- Some bony fishes have soft, flexible fin rays.
- Other bony fishes have spiny, rigid fin rays at the leading edges of the dorsal, anal, and pelvic fins.
- Both soft and spiny fin rays are modified scales.
- The spiny fin rays of some species are associated with venom glands. Fishes in the family Scorpaenidae include the stonefish (Synanceja spp.), the lionfish (Pterois spp.), and the scorpionfish (Scorpaena spp.) - some of the most venomous fishes in the world. Glands in the dorsal, anal, and pelvic spines produce venom that is intensely painful and occasionally fatal to humans.
Fishes have two kinds of fins: paired fins (pectoral and pelvic) and median fins (dorsal, caudal, and anal).
- Typically, the paired pectoral fins help a fish turn. In some fishes, pectoral fins are adapted for other functions.
- Some bony fishes, such as the hawkfishes (Cirrhitichthys spp.) use their pectoral fins to help them "perch" at the bottom and on reef areas. Mudskippers (family Periophthalmidae) support themselves on land with their pectoral fins.
- The pectoral fins of flying fishes (family Exocoetidae) are extremely long, an adaptation that allows flying fish to glide over water as far as 150 m (492 ft.) and remain airborne as long as 20 seconds.
- Some bottom-dwelling fishes such as threadfins (family Polynemidae) have taste buds and touch receptors on their pectoral fins to locate food.
- For some fishes, such as wrasses (family Labridae), pectoral fins are the main source of power for swimming.
- Paired pelvic fins add stability, and some fishes use them for slowing. In the clingfishes (family Gobiesocidae), the pelvic fins are adapted as a sucking appendage, which helps a fish hold on to stationary objects on the ocean bottom.
- The dorsal fin may be a single fin or separated into several fins. In most bony fishes, the dorsal fin is used for sudden direction changes and acts as a "keel", keeping the fish stable in the water. In some fishes, the dorsal fin is adapted for other functions.
- In the anglerfishes (order Lophiiformes), the dorsal fin is a lure that attracts prey.
- The dorsal fin of remoras (family Echeneidae) is modified into a sucking disc. Remoras cling to large fishes and mammals with this dorsal disc and are carried along as hitchhikers.
- An African knifefish (Gymnarchus niloticus) undulates its dorsal fin to move forward or backward.
- The caudal fin, or tail, is responsible for propulsion in most bony fishes. Caudal fins come in many shapes. Many continuously swimming fishes have forked caudal fins. Fishes with lunate caudal fins, such as tunas, tend to be fast swimmers that can maintain rapid speed for long durations.
- The anal fin adds stability. In some fishes, the anal fin is adapted for other functions.
- The black ghost knifefish (Apteronotus albifrons) undulates its anal fin as a means of propulsion.
- In some bony fishes, the anal fin plays a role in reproduction. The anal fin may fan sperm over eggs, or may concentrate sperm into a particular area.
- Some species of bony fishes have reduced or absent fins. For example, morays (family Muraenidae) lack pectoral fins and pelvic fins. Several species lack an anal fin.
Eye size and position vary depending on the habitat and behavior of the species.
- Some species have eyes positioned for a field of vision below or above their bodies. The South American catfish (family Hypophthalmidae) has eyes directed downward. Many species, including the sand divers (family Dactyloscopidae) have eyes directed toward the surface.
- In flatfishes in the order Pleuronectiformes, one eye migrates across the top of their skull during development. Very young juveniles are free-swimming and have an eye on each side of the head. Adults live on the sea bottom, lying and swimming on one side. The eye that would typically be on that side of the body is on the dorsal (top) of the fish, adjacent to the other eye.
In most species, the gills are protected by a flexible plate called an operculum. Most bony fishes have a single pair of gill openings. Some bony fishes such as eels (family Anguillidae) have a pair of gill holes or pores that aren't covered by an operculum.
The nostrils of most bony fishes have no connection with the mouth or gills. In some bony fishes (such as eels), the nostrils' incurrent and excurrent openings are widely separated.
Mouth shape and size are good indications of bony fish's feeding habits.
- Most bony fishes have mouths at the front end of the head.
- Some bottom-feeding species have mouths on the underside of the snout, angled toward the bottom.
- Some surface-feeding species have mouths that angle upwards.
- Butterflyfishes (family Chaetodontidae) have thin snouts and small mouths that are useful in reaching food located in crevices and cracks.
- Some species of bony fishes, like the goatfishes (family Mulidae), have fleshy barbels that fringe the mouth. These barbels can detect food.
Most species of bony fishes are covered with and protected by a layer of plates called scales.
There are four different kinds of bony fish scales: cosmoid, ganoid, cycloid, and ctenoid.
- True cosmoid scales are found only on extinct Crossopterygians. The inner layer of a cosmoid scale is compact bone. On top of this bone layer lays a spongy layer and then a layer of cosmine (a type of dentin). The upper surface is enamel. The living coelacanth has modified cosmoid scales, which are thinner than true cosmoid scales.
- Gars (family Lepisosteidae), bichirs, and reedfishes (family Polypteridae) have ganoid scales. They are similar to cosmoid scales, but a layer of ganoin (a hard, enamel-like substance) lies over the cosmine layer and under the enamel. Ganoid scales are diamond-shaped, shiny, and hard.Most bony fishes have cycloid or ctenoid scales. Both cycloid and ctenoid scales consist of an outer layer of calcium and an inner layer of connective tissue.
- Cycloid scales overlap from head to tail, an arrangement that helps reduce drag as a fish swims.
- Cycloid scales are circular and smooth. They are most common on fishes with soft fin rays.
- Ctenoid scales have a characteristic toothed edge. They are most common on fishes with spiny fin rays.
- As a fish grows, cycloid and ctenoid scales add concentric layers.
Some bony fishes may have scales only on portions of their body, and some species have no scales.
Body spines are modified scales.
Protective spines are common in slow-swimming fishes and others that need to protect themselves without moving.
Some fishes actively engage spines.
- Most surgeonfishes (family Acanthuridae) have mobile, razor-sharp precaudal fin spines that they use to protect themselves.
- The triggerfishes (family Balistidae) have three dorsal spines that lock together. These spines may allow a triggerfish to securely lodge itself between rocks and keep predators from swallowing it.
- Some pufferfishes (family Tetraodontidae) have spines that cover the entire body. The spines lie flat until the pufferfish inflates its body.
A fish secretes a layer of mucus that covers its entire body. Mucus helps protect a fish from infection.
In some bony fishes, mucus may serve additional functions.
- Some species of parrotfishes (family Scaridae) envelop their bodies in mucous bubbles at night while they rest. This mucous barrier may "hide" the parrotfish from nocturnal predators that rely on their sense of smell to locate prey.
- Young discus (Symphysodon discus) feed on the parent fish's mucus.